It frequently happens that samples being evaporated on the rotovap start to foam or bump. This is highly undesirable as it leads to sample loss and possible contamination. Why does a sample bump and/or foam and what can you do to prevent or at least minimize it?
Bumping occurs when a sample boils rapidly, forming bubbles that cause the sample being concentrated to splash out of the flask. This is normally caused by heating too quick or applying too strong of a vacuum. The chances of it causing damage and splashes into the rotovap are increased if your evaporation flask is more than half full. Go slow, start with little vacuum and visually inspect your sample while decreasing the pressure so that bumping doesn’t occur. The same goes for the water bath temperature, don’t start too hot and keep monitoring the sample while increasing the bath temperature.
While bumping can often be prevented with great care foaming is much harder to completely avoid. Foaming occurs due to surfactants being present in your sample and can be minimized by slowly applying vacuum, visually monitoring the sample, and opening and closing the stop cock to release the vacuum whenever foam appears.
There are a few things you can do to help you deal with bumping and foaming issues that may occur despite precautions. Anti-foam agents can be added to prevent foaming and a larger flask can be used to ensure that the foam stays in your evaporation flask. For bumping you can add agents, such as boiling chips, which make the boiling more uniform.
To prevent contamination and sample loss from bumping or foaming, a bump trap or a foam brake can be installed. This secondary flask will collect your sample if it bumps or foams over, avoiding product loss and a contaminated rotovap. Nowadays, more advanced rotovaps such as the RV Control V Auto Rotary Evaporator from IKA and Hei-VAP Precision by Heidolph are equipped with a sensor that will monitor the vapor pressure and slowly adjust the vacuum for you, automatically taking care of bumping and foaming without extra effort on your part.
Reducing Bumping on the Rotovap
The bump guard lesson is usually learned one of two ways: A) someone teaches you, or B) someone doesn’t. The former is ideal and you may have no idea how lucky you are. The latter looks like this…
As an undergraduate, my research project involved the multi-step synthesis of a 20-membered macrocycle. Since there were a number of steps, I performed the first reaction several times in order to stockpile a large amount of material. The first step was particularly unnerving since it required the use of 95% dry sodium (which I could feel warming it’s way to ignition in the air as I weighed it…) and the scary floor-to-ceiling solvent still.
After performing the reaction several times, I finally had enough compound to move forward. I combined all of the material and ran a large silica gel column to purify it. Given the scale, the compound eluted over a number of fractions and in a substantial amount of volume. As I added fraction after fraction to the round-bottom, I realized that it was getting pretty full. But, “what difference would a couple more fractions make?….” So I poured them in.
Eager to move on to the next step, I put the round-bottom on the rotovap (sans bump guard) and turned on the vacuum. Moments later, my world was in slow motion as I watched the explosion of my solvent out of the round-bottom and over to the collection flask. It came splashing to rest in a pool of unknown liquid in the collection flask. I stood there numb – staring at my compound like a child watching their goldfish being flushed. Although I tried to recover the compound from the collection flask, it was too dirty and too risky to carry forward in such a long synthesis. This, of course, meant I was back to square one.